Subterranean drilling systems that employ downhole drilling motors are commonly used for drilling boreholes in the earth for oil and gas exploration and production. A subterranean drilling system typically includes a downhole drilling motor that is operably connected to an output shaft. A pair of thrust-bearing apparatuses also can be operably coupled to the downhole drilling motor. A rotary drill bit configured to engage a subterranean formation and drill a borehole is connected to the output shaft. As the borehole is drilled with the rotary drill bit, pipe sections may be connected to the subterranean drilling system to form a drill string capable of progressively drilling the borehole to a larger or greater depth within the earth.
Each thrust-bearing apparatus includes a stator that does not rotate relative to the motor housing and a rotor that is attached to the output shaft and rotates with the output shaft. The stator and rotor each include a plurality of bearing elements that may be fabricated from polycrystalline diamond compacts (“PDCs”) that provide diamond bearing surfaces that bear against each other during use.
In operation, high-pressure drilling fluid may be circulated through the drill string and power section of the downhole drilling motor, usually prior to the rotary drill bit engaging the bottom of the borehole, to generate torque and rotate the output shaft and the rotary drill bit attached to the output shaft. When the rotary drill bit engages the bottom of the borehole, a thrust load is generated, which is commonly referred to as “on-bottom thrust” that tends to compress and is carried, at least in part, by one of the thrust-bearing apparatuses. Fluid flow through the power section may cause what is commonly referred to as “off-bottom thrust,” which is carried, at least in part, by the other thrust-bearing apparatus. The on-bottom and off-bottom thrust carried by the thrust-bearing apparatuses can be extremely large.
The operational lifetime of a subterranean drilling system can often be determined by the useful life of the thrust-bearing apparatus. Therefore, manufacturers and users of thrust-bearing apparatuses continue to attempt to improve their operational lifetime.